For the sausages, melt the butter in a large non-stick frying pan and fry the leek gently for 8-10minutes, or until very soft but not coloured.
Put 100g/3½oz of the breadcrumbs, the parsley, thyme and cheese in a large mixing bowl and mix until well combined. Beat the egg yolks with the mustard, salt and plenty of freshly ground black pepper in a separate bowl.
Remove the frying pan from the heat and tip the leeks into the bowl with the breadcrumbs. Add the egg yolks and mix together well with a large wooden spoon until well combined. Divide the leek mixture into eight portions and roll into sausage shapes. Place the sausages onto a tray lined with clingfilm.
Whisk the egg whites lightly in a bowl with a large metal whisk until just frothy. Sprinkle 40g/1½oz breadcrumbs over a large plate. Dip the sausages one at a time into the beaten egg and roll in the breadcrumbs until evenly coated, then place on the baking tray. Chill the sausages in the fridge for 30 minutes.
Heat the oil into a large non-stick frying pan and fry the sausages over a medium heat for 10-12 minutes, turning regularly until golden-brown and crisp. Then Serve!